Field of the Invention
The present invention relates to wireless telecommunications networks, and more specifically relates to systems, equipment, components, software and methods for troubleshooting signals in cellular communications networks.
Description of the Prior Art
A) Overview of Wireless Telecommunications Networks
FIG. 1 shows an overview of a typical wireless telecommunications network 2. To facilitate an understanding of the invention, the steps in carrying on a conversation between New York and California on a wireless cellular network 2 will now be explained. When the person in New York inputs on his cellular phone 4 the number of the person in California and presses “call” or “send”, a process is started to find the person in California and send a message to them to make his phone ring. When the person in California answers the call, a transmission path is set up to send and receive their conversation across the country.
For the purposes of this invention, the details of how the phone conversation is set up need not be described. This present invention is concerned with enabling the accurate recovery of a transmitted message in the section 6 of the network 2 which is linked by a radio transmitter and radio receiver. This section 6 of the network 2 is called the “Radio Access Network” which is commonly abbreviated as “RAN”. For purposes of illustration, we will describe a voice conversation. However, the same concepts apply to any other radio transmission (data, video, etc.)
B) The Radio Access Network (RAN) is the Weakest Link in a Wireless Telecommunications Network
Telecommunications is a chain of transmit and receive processes. In the case of voice conversations, human speech is received by a microphone and converted to analog signals (modulation of electromagnetic force (changes in voltage with respect to time)). The analog signals are converted to a digital representation in an analog-to-digital converter and then the digits (1s and 0s) are transported over a distance to a receiver where the 1s and 0s are converted from digital back to analog and presented to a person via a speaker. If the digital signal (1s and 0s) is not received exactly as it was transmitted, then there is distortion in the audio signal, and the person at the receiving end may not understand the conversation.
Referring again to FIG. 1, each connection between a transmitter and a receiver is commonly referred to as a “hop”. An end-to-end connection consists of several hops, each of which must correctly transmit and receive the data, through multiple Mobile Switching Centers (MSC) 7. The limiting factor in the network equipment's ability to accurately recover the signal is the signal to interference plus noise ratio (“SINR”) at the receiver. Every receiving device has an SINR at which it can no longer correctly recover the signal that was sent by the transmitter. Mathematically, the signal to interference and noise ratio is expressed as:
  SINR  =            Signal      ⁢                          ⁢      level              Interference      +              Noise        ⁢                                  ⁢        level            
where the level (amount) of the signal and the level of the noise are measured in the same units (usually power, expressed in Watts).
For each hop in the telecommunications network 2, the path between the transmitter and the receiver is called the “transmission medium” 8. In the mobile phone network 2, the transmission mediums are:                1. Transmission of pressure waves from the lips of a human talker to the microphone of a mobile phone 4 a short distance through earth's atmosphere.        2. Transmission of radio waves over-the-air from the mobile phone 4 to the radio receiver in the network 2 over distances up to approximately 10 kilometers.        3. Transmission of pulses of light through strands of glass (fiber-optic cables) 8 over distances up to approximately 100 kilometers.        4. Transmission of electrical force over electrical conductors over short distances (meters).        5. Transmission of pressure waves from the speaker of a mobile phone to the eardrums of a human listener a short distance through earth's atmosphere.        
The environment in which pressure waves are transmitted by a talker to a microphone (item 1) and from a speaker to a listener (item 5) can be a significant source of distortion in the quality of the end-to-end conversation (example: talking or listening in a crowded, noisy room). However, this SINR environment is outside the control of the Wireless Service Provider (WSP), so it is not a process the WSP tries to quantify, measure, and manage.
The transmission of electrical signals in the network (item 4) occurs over short distances (usually along a circuit path inside a piece of equipment or short distances between pieces of equipment) and are generally near 100% reliable (literally greater than 99.999% reliability).
Transmission of light pulses through fiber-optic cables occurs over long distances, but the transmission medium 8 is very good. The characteristics of the fiber-optic cables are very well known and are very stable (i.e. the characteristics have very low variability). Therefore, even though transmission of light pulses over fiber-optic cable 8 covers long distances, it can be engineered to consistently provide greater than 99.999% reliability.
The transmission medium 8 in which almost all the problems occur is the over-the-air radio wave environment. While the characteristics of radio wave transmission in free space (e.g., between the earth and the moon) are very well understood, the transmission of radio waves in the mobile phone network 2 can only be predicted statistically. There are several reasons for this:                1. The radio waves transmitted by the mobile phone 4 to the network receiver are relatively low power.        2. Radio wave transmission in the mobile phone environment does not occur in free space—it occurs in space that is filled with reflectors and absorbers (buildings, cars, people, etc. . . . ). This causes high variability in the signal level part of the SINR equation.        3. Radio waves must be transmitted on specific frequencies. If a device outside the control of the wireless service provider is broken and transmitting radio energy on the same frequency as the wireless service provider's network receiver, then this causes high variability in the interference part of the SINR equation.        4. The equipment that is used to transmit and receive radio waves over-the-air is exposed to harsh weather conditions while the equipment used for fiber-optic transmission is housed in environmentally-controlled offices. Therefore, the radio transmission and reception equipment is more prone to degradation and failure.        
The net result is that the over-the-air radio transmission environment has high variability in the signal part (numerator) of the SINR equation and sometimes also has high variability in the interference and noise part (denominator) of the SINR equation.Signal-to-Interference+Noise Ratio (SINR) for transmission of digital signals over fiber-optic lines:
            SINR      ⁡              (        fiber        )              =                  predictable        ⁢                                  ⁢        Signal                    predictable        ⁢                                  ⁢        Noise        ⁢                                  ⁢                  (                      and            ⁢                                                  ⁢            no            ⁢                                                  ⁢            interference                    )                                Signal      ⁢              -            ⁢      to      ⁢              -            ⁢      Interference        +          Noise      ⁢                          ⁢      Ratio      ⁢                          ⁢              (        SINR        )            ⁢                          ⁢      for      ⁢                          ⁢      transmission      ⁢                          ⁢      of      ⁢                          ⁢      radio      ⁢                          ⁢      signals      ⁢                          ⁢      over      ⁢              -            ⁢      the      ⁢              -            ⁢      air      ⁢              :                        SINR      ⁡              (        RAN        )              =                  unpredictable        ⁢                                  ⁢        Signal                              unpredictable          ⁢                                          ⁢          Interferene                +                  Noise          ⁢                                          ⁢                      (            sometimes            )                              
The signal to noise ratio of the fiber-optic (and electrical) part of the network 2 is highly predictable, and the signal-to-noise ratio of the Radio Access Network 6 is highly unpredictable. If the variability is low, then telecommunications engineers can accurately design the system for high reliability. If there is high variability in the system, it is much more difficult to achieve high reliability. This is why the RAN environment is always the limiting factor in the reliability of mobile telecommunications networks.
C) Frequency Spectrum is a Scarce and Valuable Resource
In addition to the reliability problems, there are also capacity constraints in the RAN environment. The capacity for data transfer (measured in bits per second) over a fiber-optic line is vastly greater than the data transmission capacity of the RAN environment.
Frequency spectrum is a shared public resource that is regulated and controlled by governmental agencies (the Federal Communications Commission in the United States). The FCC auctions licenses to operate in defined frequency ranges to the wireless service providers. The frequency spectrum of the RAN environment is a precious resource because there is a finite supply. Because there is a limited supply of frequency spectrum, and because of the growth in demand for wireless services by consumers, the cost for these licenses has risen dramatically. The most recent frequency auction in the United States garnered $45 billion dollars for the right to use 50 MHz (megahertz) of frequency.
D) Summary of Key Points about the RAN:
                1. The signal-to-noise ratio (SINR) at the most disadvantaged receiver is the limiting factor in any telecommunications system.        
  SINR  =            signal      ⁢                          ⁢      power      ⁢                          ⁢      level              interference      +              noise        ⁢                                  ⁢        power        ⁢                                  ⁢        level                                                When SINR goes down, reliability, capacity, and data throughput all go down.                            When signal power goes down, SINR goes down.                When noise or interference goes up, SINR goes down.                                                2. The most disadvantaged receiver in all wireless telecom networks 2 is the receiver in the network 2 that must recover the radio signal transmitted by the mobile phone 4.                    The transmit power of the mobile phone 4 cannot be increased because of safety concerns and because of practical limitations on size and battery power. Mobile phones 4 are limited by regulation to a maximum transmit power of less than ½ of a Watt (by contrast, microwave ovens operate at about 500 Watts).                        3. The variability of the signal power level that is received by the wireless telecommunications network 2 is very high and cannot be controlled by engineers.                    The signal that is transmitted by the mobile phone 4 is subject to conditions (reflection, absorption, and scattering) in the RAN environment that are well understood, but can only be predicted using statistical models.            The interaction of these effects is called “fading” and can result in temporary reductions of the signal level by factors of 10 to 100 (i.e. over a short period of time the signal level can be as little as 1/100th of the normal signal level).            The technique used to reduce the variability of received signal level is the use of multiple receive antennas 10 (called “diversity antennas”). This technique works because the probability is very low that both antennas 10 will experience fading by the changing RAN environment at the same time.                        4. Under normal operating conditions, the noise power level (in the denominator of the SINR equation) is predictable and the interference is non-existent; however, there are often problems in the RAN environment that cause the interference and noise power level to be unpredictable. The main source of these problems is equipment degradation due to exposure to weather conditions and interference from other transmitters.        5. Engineers can design a system that accounts for the variability of the received signal power level and provides reasonably good reliability. However, because of zoning restrictions and practical economic and construction limitations, wireless service providers cannot put receivers everywhere they want. Therefore, wireless telecommunications networks 2 often operate near the reliable limits of transmission based on the SINR. If a connection between a mobile phone transmitter and the network receiver is operating near that limit and the interference or noise level rises, the radio link can become unusable; then speech becomes garbled or in the worst case the call drops.E) Problem Conditions in the Radio Access Network (RAN)        
Two problems which reduce the reliability and capacity of the wireless telecommunications network 2 are breakdowns in the balance of the diversity antennas 10, and increases in the noise level at the radio receiver. The equipment in the network 2 monitors for these conditions and sends notifications when problems are detected. The generic terms for these notifications are:                Diversity antenna imbalance alarm, when the signal strength from the multiple receive antennas 10 is significantly different for a sustained amount of time. While fading can cause short term differences, if the difference is large and stays for a while, something else is causing the problem. The most common causes are a failed antenna or connecting cable in one branch. These can be outright failures, such as a broken cable, but are more commonly a subtler problem such as Passive Intermodulation Distortion.        High RSSI (Received Signal Strength Indicator). High RSSI is misleading because it seems like high signal strength would be good, not bad. The reason that high RSSI is detected as a possible problem condition is:                    The mobile network 2 sends power control commands to the mobile phone 4 to power down the mobile phone's transmitter to a level that is strong enough to maintain a SINR that enables good quality reception but not stronger than necessary. Keeping the transmit power level of the phone 4 as low as possible while still maintaining a good SINR has two advantages:                            It preserves the battery life in the mobile phone 4.                Often, especially in dense urban environments, there is more than one network receiver that detects the RF energy that is transmitted by the mobile phone 4. In those circumstances, the RF energy from the mobile phone 4 is considered noise by the other receivers.                                    In most cases, the reason that RSSI is too high is because there is unwanted noise or interference coming into the receiver. This causes the network 2 to command the mobile phone 4 to transmit at a higher than normal level in order to maintain a good SINR.F) Root Causes of the Problems                        
There are many potential causes of problems in the RAN 6, but the common of them are:                Interference from an external transmitter (one that is not under the control of the wireless service provider) that is broken and creating noise in the wireless service provider's frequency spectrum. This is commonly called “interference”.        Interference from Passive Intermodulation Distortion (“PIM”) that is produced by a bad connector or loose connection in an antenna. This is called “internal PIM” if the source of the PIM is in the network equipment up to and including the antenna 10.        Interference from Passive Intermodulation Distortion (“PIM”) that is produced by a semi-conductor that is external to the network equipment. This is called “external PIM”. The source of external PIM is usually a rusty surface that mixes the RF energy transmitted by the radiating elements in the antenna 10 and reflects it back to the receiving elements in the antenna 10.        An unmanaged repeater, also known as a bi-directional amplifier or BDA that has gain that is too high, causing signals from a cell phone 4 to appear too high at the base station 12.        
The root causes, physical manifestations, and alarms and indications of the physical manifestations are summarized in Table 2. The key point of the table is that the alarms and notifications are generally insufficient by themselves to diagnose and repair the root causes of the problems.
G) Diagnosing the Root Causes
A typical procedure for diagnosing the root causes requires:                A spectrum analyzer hooked up to RF monitor port at the base station.                    A knowledgeable technician or RF engineer on site while the problem is occurring. However, problems are often intermittent (i.e. PIM only when it is windy or intermittent interference). It is like the gremlin in your car that does not show itself when you take it to the repair shop—you know something is wrong but you cannot diagnose it, so you just start changing parts and hope the problem goes away.                        
There is a high cost of mis-diagnosis:                Direct cost: wireless service providers spend thousands (sometimes 10s of thousands) of dollars changing antennas and/or transmission lines only to find that the repair did not fix the problem. (Evidence=no fault found in the replaced components, or no improvement in RAN performance.)        Indirect cost: money and man-hours that could have been spent building out the new network.H) Change in Radio Access Network Architecture in Wireless Telecommunications Networks        
The Wireless Telecom Network 2 is currently undergoing a transition in the architecture of the radio access network (aka RAN 6). The traditional RAN architecture (which has been used since the 1980s) employed equipment in which the electronics were housed in a controlled environment and the radio signal was sent and received over a coaxial transmission line 14 to an antenna 10 which transmitted and received the radio signal over the air to mobile phones 4. A typical embodiment of this architecture is shown in FIG. 2. Throughout the rest of this disclosure, the inventors will refer to the traditional RAN or T-RAN for short.
The modern approach splits the function of the base station 12 into two pieces of equipment, called the Radio Equipment (“RE”) 16 and the Radio Equipment Controller (“REC”) 18 as shown in FIG. 3. The RE 16 and the REC 18 can be separated by an arbitrary distance. For example, with this technology is used at an independent tower location, the RE 16 is usually mounted near the top of the tower and the REC 18 is at the bottom of the tower. There is also a new architecture called C-RAN (Cloud or Centralized RAN), in which several RECs 18 are housed in a central location and the REs 16 are connected to them over distances up to 15 kilometers.
The Radio Equipment 16 transmits the radio signal to the mobile phones 4 and receives signals from those mobile phones 4. The Radio Equipment 16 may have multiple transmitters and receivers at the same frequency, for diversity or so-called MIMO (Multiple Input Multiple Output) functions. The Radio Equipment Controller 18 processes the baseband modulation data (in the mathematical format of “I/Q vectors”, where “I” represents the in-phase signal component and “Q” represents the quadrature phase signal component).
In the C-RAN architecture shown in FIG. 3 and FIG. 4, the REC 18 and the RE 16 have a digital data connection that can be extended up to 15 kilometers (about 10 miles) over a highly reliable fiber-optic connection 20, often using an interface called the Common Public Radio Interface (“CPRI”). Hundreds of REs 16 can be connected to the REC equipment 18 that is housed in one location. This is why the C-RAN architecture is sometimes also referred to as “Base Station Hoteling”.
There are three primary reasons that the Wireless Service Providers are investing in the C-RAN architecture:
1. Improvements in spectral efficiency.
2. Reductions in real estate and utility costs.
3. Improvements in quality of service to their customers.
I) The RE/REC Technology Enables Automated, Remotely-Controlled Diagnosis of the Root Causes of Problems in the RAN Environment
The CPRI connection between RE 16 and REC 18 employs fiber-optic transmission lines 20 to transport I/Q data. I/Q data is to radio frequency modulation what an MP3 is to music—it is the digital representation of the analog modulation (change in voltage with respect to time).
In the downlink communication channel (from the network 2 to the mobile telephone 4), the I/Q data has no distortion, because at the point that it is observed, it has not yet been subjected to the effects of the RAN environment or any other sources of distortion.
In the uplink communication channel, the I/Q data contains the signal created by the phone 4 plus the effects of the RAN environment (path loss and fading effects) and distortion from noise sources (the problems the RANALYZER™ system of the present invention is designed to diagnose). In the uplink direction, the REC 18 processes the I/Q data and attempts to recover the original signal (in the presence of noise) as transmitted by the phone 4.
The RANALYZER™ system of the present invention processes the IQ data and attempts to separate out the noise component (in the presence of signal) to determine the root cause (source) of the noise. The methods for separating out the noise from the signal and analyzing the noise to discover its source, in accordance with the present invention, and the RANALYZER™ system 22 of the present invention, will now be disclosed.